Ohmmeter



A. G. ASAFF Oct. 26, 1954 OI-IMMETER Filed June 27, 1950 Patented Oct. 26, 1954 iran ATENT OFFICE Callaghan Hession Corporation,

Boston, Mass.,

a corporation of Massachusetts Application June 27, 1950, Serial No. 170,671 2 Claims. (Cl. 324-62) This invention relates to a precautionary safety device for indicating the capacity of persons to accumulate electrostatic charges by measuring their individual electrical resistance while in contact with a conductive ooring. -In particular it comprises a meter adapted to measure the electrical resistance between a person and ground quickly, conveniently and with a minimum of interference with his occupational routine.

It is often necessary for persons to work under conditions where explosive concentrations of combustible material are likely to accumulate in the atmosphere. A dangerous condition results when such persons accumulate electrostatic charges, because the spark caused by the discharge of such charges is likely to initiate an explosion. It is, therefore, common and desirable to maintain a path of low resistance between the person and ground in order that electrostatic charges which are generated will be dissipated by a dow of current to ground. Hospital operating rooms, where there is often the danger of explosions from the vapors of certain modern anaesthetics, particularly cyclopropane, are frequently provided with semi-conductive ooring material and attempts are made to keep personnel and equipment in substantial electrical contact with this ooring. The effectiveness of such precautionary measures cannot be known, however, unless an actual indication or measurement of the .resistance between a person and ground be made. Doctors and other hospital personnel, for instance, may pick up a layer of wax on the soles of their shoes that prevents electrical contact from being made with the conductive flooring.

The indication or measurement of the resistance between a person and ground should be made under actual working conditions in order to determine his tendency then and there to accumulate a charge, and minimize the effect of intervening events which may interfere with the conductive path. It should also be done quickly and conveniently so as not to interfere materially with his working routine, as experience shows that if the measurement is too involved the personnel will become indifferent to it. And in the case of hospital operating room personnel, the measurement must be made without contaminating their surgically clean hands. The measuring device should also be accessible to all persons, regardless of physical stature, so that only one'device need be provided in each locality.

The principal object of this invention is to provide an instrument for measuring the resistance between a person and ground under actual work- 2. ing conditions with an absolute minimum of interference with his working routine, accessible to all, and without requiring manual contact.

This invention is essentially a meter having a dial calibrated to read resistance. It is well known that an ohmmeter must utilize a source of electromotive force since the measurement is essentially one of determining the voltage drop and current ratio. One embodiment of this invention utilizes the /120 volt utility system, supplemented by a transformer and rectified, as the source of electromotive force. One feature of this embodiment of the invention is that it provides an instrument which draws power only when a measurement is being made, the making of measuring contact turning the power on and the breaking of measuring contact turning it to-73 The National Fire Protection Association has set an upper limit of 500,000 ohms resistance between the person and ground as safely preventing the accumulation of dangerous charges of electrostatic electricity. This invention comprises an ohmmeter adapted to measure resistances of the order of magnitude of 500,000 ohms, one measuring terminal of which is grounded or otherwise electrically connected to the common potential level of the working area. IThe other terminal is connected to a conductive arm which projects from the meter in such a manner as to be readily accessible to non-critical regions of the body, as for instance, the elbow. This arm may be adapted to be moved slightly by the pressure exerted against it and connected with a switch in the main power circuit to the meter. Pressure against the arm, made for taking the measurement of ones resistance to ground, displaces the arm and closes the switch thereby turning the meter on only during the period of the measurement.

These features and other advantages will be best understood from the following description of a preferred embodiment of this invention, selected for illustrative purposes and not by way of limitation, and from the accompanying drawings whereto reference is made in which:

Fig. 1 is a front view of the meter mounted on a wall,

Fig. 2 is a cross sectional View showing the conductor arm and switch, and

Fig. 3 is the diagram of the circuit used in the ohmmeter.

The'ohmmeter is one of conventional direct reading design as shown in Fig. 1. Power from an ordinary 110/ 120 volt source is supplied to the terminals I and Il, terminal I0 being the line ground. The line from terminal I I connects with a fuse I2 and then with switch I3 which is normally open. The circuit is completed through primary windings I4 of an electrostatically shielded transformer I5 giving a secondary voltage of 90 volts. A lamp I6 is placed in parallel with the primary windings I4 and serves to indicate when power is being supplied to the meter. The secondary windings II of transformer I5 are part of a rectifier circuit comprising in series the secondary windings I'I, a 50-ohm resistor I8, a (selenium) rectiiier I9 and a 50 microfarad filter condenser which is shunted by a 1 megohm resistor 2l. The negative side of the rectifier circuit is connected through a 250,000 ohm current limiting resistor 22 to the ground terminal 23 of the meter. This connection is made through the terminals of a jack 29.

The positive side 2li of the rectifier supplies an electrornotive force of about 90 volts to the ohmmeter circuit. The National Fire Protection Association recuires that resistance measurements such as these be made at potential of from 90 to 500 volts, and at currents of less than 10 milliainps, alternatively the power supply can consist of any source oi D. C. voltage of from 90 to 500 volts in series with a resistance calculated to limit the current to less than 10 milliamps. The high resistance of the source of potential simulates electrostatic voltage conditions in that the high voltages are accompanied by low currents, rendering it safe to apply the measuring voltage of from 90 to 500 Volts across a human body.

The ohrnmeter circuit comprises a microammeter 25o having a range of about Li0@ microamperes and an armature resistance of 65 ohms, which is paralleled by the series circuit comprising a variable 2000 ohm resistor 25 and a 200 ohm resistor 25. The range oi the microarnrneter ultimately depends on the measuring voltage and resistance ci the circuit. The microammeter 26s'. is calibrated to read directly in ohms. lThe unknown resistance oi ones body to ground is connected in series with the ohrnmeter circuit through the lead 2l which is connected with a downwardly projecting conductive arm 23. Body contact is made with this arm thereby completing the circuit by connecting, through the body resistance to ground, the conductive arm the ground terminal 23.

A jack 20 is provided for ing an integral normally open switch Sli. The collar and tip connections ci the jack connect respectively to ground and the measuring terminal. The switch connections parallel the connections oi switch I3. insertion of a plug connected with known resistances, urns the meter on and provides a convenient way of Calibrating and testing the resistance cetween any point on the floor and ground.

For most applications oi this invention the ground 23 will be an earth ground, such as is obtained by connecting the terminal to a rod driven well into the earth or to a cold water main. In some applications however true ground potential may have no signiiicance, as when the entire working area in which the ohrnmeter is installed is insulated from ground. In such cases terminal II should be connected with an object which is itself at the common potential level, as e. g. all personnel may come in contact.

When a rectified A. C. source of voltage is used, one embodiment of this invention provides the testing purposes havconductive objects with which switch I3 for turning the power on and ofi which is actuated by the making of measuring contact with the conductive arm 28. This feature, as well as the details of the conductive arm, are shown in Fig. 2. Switch I3 is mounted on a platform 3| having an aperture through which the push button 32 of switch protrudes. The leads 33-33 of the switch connect with the fuse I2 and primary I4 of transformer I5 shown in Fig. i. Platform 3l is mounted on sleeves 34 which rest against the rear side of the face plate 35. The screws 3S secure the platform 3I and switch I3 in their proper functional relationships. The conductor arm 28 is fastened pivotally by means oi pin 31 to a base 33 of electrically conductive material having a recessed portion adapted to receive the upper end of the arm. A spring 35 yieldingly holds the arm 2S outwardly away from the base 39. The base with the accompanying arm 23 is held to the face plate 35 by means of the screws 36. The base and arm 23 as thus mounted oppose the switch I3 and its push button 32. A rod I0 extends through a hole in the face plate 35 and through a hole in the base 38. One end is in contact with the arm 28 and the other end engages the push button 32 of the switch. Depression of the arm 23 causes it to rock slightly about the pivot 31 and push the rod itil and the push button 32, thereby turning the meter on Breaking contact permits the spring 39 to urge the arm 28 bach to its normal position, and a spring (not shown) in the switch returns the push button 32 and rod :l0 to their normal positions, breaking the circuit.

The conductive arm 20 is electrically incorporated in the ohmrneter circuit through the lead il which is held under the head of one of the screws St. This screw electrically connects the lead di with the base 33 and conductive arm 28 with which the base is in contact.

The meter is mounted for operation as shown in Fig. 3, conveniently on a wall with the conductive arm 2B extending downward. The dial 42 of the meter is conveniently placed just above the conductive arm 28 so that a person can read his own resistance measurement. Conveniently the scale of the dial Q2 can be provided with a color code. For instance that portion of the dial indicating less than 500,000 ohms can be green, indicating that the resistance of the person being measured is within safe limits. rihat portion from 1,000,000 ohms to 500,000 ohms can be orange, indicating that the resistance is questionable, and that portion indicating resistances greater than 1,000,000 can be red, indicating that the resistance is dangerously high. A control screw i3 oi the variable resistor 26 shown in Fig. 1 protrudes through the panel 35 to be readily accessible ior calibration. A jack M and the indicating light I0, both shown in Fig. 1, are also mounted on the face plate 35.

It will be understood that the ohmmeter is usually mounted on the wall of an operating room at a height convenient to be reached by the bared elbow of a nurse or doctor so that, in approaching the operating table, he or she may depress the arm 20 and get an instantaneous indication from the meter as to the degree of insulation or resistance to the discharge of static charge. With this arrangement it is not necessary or the person to contact a sterilized glove with the meter in order to get the desired indication. Moreover snce the meter may be mounted above the four foot level it is not subject to the vapor-proof requirements of the underwriter.

It will be understood that three test circuits may be utilized in employing the ohmmeter above described; 1) body to iiooring using the switch a-rm 28, (2) any point from floor to ground using the jack t4, or (3) any two points in the flooring using the jack and a second electrode.

Having thus disclosed my invention and described in detail an illustrative embodiment thereof, I claim as new and desire to secure by Letters Patent:

1. An ohmmeter for measuring the resistance between a person standing on a conductive flooring and ground, including a terminal for a rectifier circuit having connection with a source of A. C. voltage and a normally open switch in said circuit adapted to interrupt the flow of alternating current to the rectifier, an ohmmeter circuit in series with said rectier circuit, said ohmmeter circuit including a resistance-indicating meter and terminals for connection across the unknown resistance, one terminal being grounded and the other connected with a downwardly protruding arm of conducting material, adapted to make electrical Contact with bare regions of the body without manual contact, pivotally mounted at its upper end and yieldingly held in a normally Vertical position subject to angular dis" placement about its pivot by linear force, said arm communicating mechanically with said switch whereby displacement of the conductive arm closes the switch.

2. An ohmmeter for measuring the resistance between a person standing on a conductive i'looring and ground, including a terminal for a rectier circuit having connection with a source of A. C. voltage and a normally open switch in said circuit adapted to interrupt the iow of alternating current to the rectifier, an ohmmeter circuit in series with said rectifier circuit, said chinmeter circuit including a resistance-indicating meter and terminals for connection across the unknown resistance, one terminal being grounded and the other connected with a downwardly protruding arm of conductive material, adapted to make electrical contact with bare regions of the body without manual contact, pivotally mounted at its upper end and yieldingly held in a normally vertical position subject to angular displacement about its pivot by linear force, said arm communicating mechanically with said switch whereby displacement of the conductive arm closes the switch, and said ohmmeter circuit including jack connections for changing the circuit ground to another conductive object.

References Cited in the me of this patent UNITED STATES PATENTS Number Name Date 629,022 Wolinski July 18, 1899 786,533 Sutton et al Apr. 4, 1905 1,088,780 Katz Mar. 3, 1914 1,593,024 Macadie July 20, 1926 1,593,029 Pagliarul July 20, 1926 1,593,638 Moyer July 27, 1926 1,764,410 Legg June 17, 1930 2,157,973 Wenger May 9, 1939 2,556,458 Webster June 12, 1951 

